S-53

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The Tektronix S-53 Trigger Recognizer head can be used in the right bay of the 7S12 TDR/Sampler plug-in
or externally in a type 285 enclosure. It essentially contains the entire trigger circuit of an oscilloscope.

The S-53 uses tunnel diode triggering with an amplifier before the tunnel diode.

The signal first passes through a compensated NPN differential amplifier, Q10 and Q12, which provides
an adjustable trigger offset voltage and trigger polarity selection. Switching diodes CR6 and CR8 are
used for selecting the trigger polarity. The differential amplifier (i.e., phase splitter) generates
both phases, inverted and non-inverted. Depending on the position of the trigger polarity switch, one
or the other of the phases is shunted by 200 Ω resistor. The two signals are buffered by common-base
amplifiers Q20 and Q22, whose collector currents are summed. R25 sets up a bias current on trigger
tunnel diode CR25. The extra current from Q20 or Q22 pushes CR25 over its peak current, and it switches
to the high voltage state.